1. Field of the Invention
This invention pertains to the general field of optical metrology. In particular, it pertains to a pneumatically counterbalanced gantry suitable for manual positioning of an interferometric objective in precise alignment with a large sample object.
2. Description of the Prior Art
Mounting an interferometric optical head on a gantry-type structure makes it possible to measure large parts with oddly placed surfaces or recesses, such as engine parts. However, the process of alignment of the optical head with the test surface requires a versatility of movement that is not available in conventional instrumentation. Interferometric objectives have to be roughly perpendicular to the surface of interest in order to perform a measurement and the small angular range of adjustment of conventional stages is critical for fine tuning but is not capable of providing the large range of motion required for aligning large samples.
In order to accommodate such large parts, the optical head is usually mounted on a gantry equipped with a large-angle tilting mechanism that allows rotation of the entire objective assembly through approximately 90 degrees (+/−45 from vertical) of motion. Such a gantry arrangement makes it possible to measure irregularly shaped samples without the use of costly and cumbersome part fixtures dedicated to specific parts. The tilting mechanism may be implemented simply with two bearing spindles mounted coaxially on opposite sides of the optical head assembly for rotation within the gantry. The center of rotation is advantageously located at a convenient position with respect to the stage.
The gantry is preferably movable vertically along two supporting rail structures, one on each side of the gantry, in order to provide vertical adjustment of the optical head position with respect to the test object. Thus, in preparation for a measurement, the gantry is lifted to provide a safe clearance for the measurement objective while the large test object is placed in its measurement stage, then the gantry is lowered and the optical head is tilted to the degree necessary to provide the desired alignment for the measurement.
This arrangement works well in theory, but in practice it presents several operating problems that render it impractical for either manual or automated operation. The large range of vertical motion desired for the gantry, so that it can accommodate parts of very different heights, makes motor-driven adjustments impractical because they are relatively slow and costly, and they would require a sophisticated crash-control mechanisms. Manual adjustment is therefore preferred, but the considerable weight of the gantry requires that a substantial upward force be applied to lift it, as well as a substantial support force be applied while lowering it in order to control its descent to the degree necessary to avoid crashing the head into the part. Therefore, manual operation of this type of gantry is impractical as well. This invention describes a pneumatic counterbalance mechanism that makes it possible to manually adjust the position of the optical head rapidly and with precision.